Systems and methods for low power pulse oximetry

ABSTRACT

Methods and systems are provided for lowering power consumption in an optical sensor, such as a pulse oximeter. In one example, a method for an optical sensor includes illuminating a light emitter of the optical sensor according to set sensor parameters, the sensor parameters set based on hardware noise or external interference characterization and light transmission or reflection of a tissue contributing to a signal output by the optical sensor, the sensor parameters including current drive parameters of the light emitter, and adjusting the current drive parameters of the light emitter to maintain a target signal to noise ratio of the signal output by the optical sensor.

FIELD

Embodiments of the subject matter disclosed herein relate to biologicalprobes, sensors, and methods, and in particular, to photoplethysmographyprobes, sensors, and methods.

BACKGROUND

Photoplethysmography (PPG) relates to the use of optical signalstransmitted through or reflected by blood-perfused tissues formonitoring a physiological parameter of a subject (also referred to as apatient herein). In this technique, one or more emitters are used todirect light at a tissue, and one or more detectors are used to detectthe light that is transmitted through or reflected by the tissue. Thevolume of blood of the tissue affects the amount of light that istransmitted or reflected, which is output as a PPG signal. As the bloodvolume in a tissue changes with each heartbeat, the PPG signal alsovaries with each heartbeat.

Pulse oximetry is, at present, the standard of care for continuouslymonitoring arterial oxygen saturation (SpO₂). Pulse oximeters includePPG sensors that use red (˜660 nm) and infrared (˜940 nm) light todetermine physiological parameters (e.g., blood characteristics) of thesubject, including SpO₂, pulse rate, and pulsating blood flow (e.g.,blood perfusion) at the site of measurement. Conventional pulse oximetrysensors are typically mounted to an extremity of the subject (e.g., afinger or ear lobe).

BRIEF DESCRIPTION

In one embodiment, a method for an optical sensor includes illuminatinga light emitter of the sensor according to set sensor parameters, thesensor parameters set based on hardware noise or external interferencecharacterization and light transmission or reflection of a tissuecontributing to a signal output by the optical sensor, the sensorparameters including current drive parameters of the light emitter, andadjusting the current drive parameters of the light emitter to maintaina target signal to noise ratio of the signal output by the opticalsensor.

Thus, current drive parameters of the light emitter, which may includethe current pulse amplitude as well as pulse length and pulse frequency,may be selected based on hardware noise and ambient noise thatcontribute to the signal output from the sensor. By selecting thecurrent drive parameters according to measured and/or estimated noisecontributions, and then adjusting the current pulse amplitude tomaintain the target signal to noise ratio of the signal, sufficientsignal quality may be maintained to allow accurate measurement ofphysiological parameters, while operating the probe at a lowest possiblepower consumption.

It should be understood that the brief description above is provided tointroduce in simplified form a selection of concepts that are furtherdescribed in the detailed description. It is not meant to identify keyor essential features of the claimed subject matter, the scope of whichis defined uniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from reading thefollowing description of non-limiting embodiments, with reference to theattached drawings, wherein below:

FIG. 1 is a block diagram illustrating an example pulse oximetry system.

FIG. 2 schematically shows an example drive circuit for a pulse oximetrysystem.

FIG. 3 is a flow chart illustrating an example method for characterizinghardware noise in a pulse oximeter.

FIG. 4 is a flow chart illustrating an example method for lowering powerconsumption of a pulse oximeter during use.

FIG. 5 shows an example relationship between signal-to-noise ratio andaverage LED current.

DETAILED DESCRIPTION

The following description relates to various embodiments of an opticalsensor. The sensor may be included in a pulse oximetry sensor or system,such as the system of FIG. 1, for determining physiological parametersof a patient. The optical sensor may include two light emitters, hereinlight emitting diodes (LEDs), driven by a drive circuit, such as thedrive circuit illustrated in FIG. 2. Hardware noise of the pulseoximetry sensor may be measured and/or estimated at time of manufacture,as shown by the method of FIG. 3. In particular, the sensorsignal-to-noise ratio may be characterized at a plurality of average LEDcurrents for a plurality of current transfer ratios. Then, during sensorinitialization, an LED current pulse amplitude may be adjusted tomaintain a desired signal-to-noise ratio, such as according to themethod of FIG. 4. FIG. 5 shows a diagram relating the signal-to-noiseratio to average LED current, which may vary for different pulseoximeters. The desired signal-to-noise ratio may be determined based onthe measured and/or estimated hardware noise and may be further adjustedbased on hardware limits. In the way, the hardware parameters thatimpact power consumption may be adjusted in a manner that consumes alowest amount of power while still providing a suitable signal quality.

A pulse oximeter comprises a computerized measuring unit and a probeattached to a patient, typically a finger or ear lobe of the patient.The probe includes a light source for sending an optical signal throughtissue of the patient and a photo detector for receiving the signaltransmitted through or reflected from the tissue. On the basis of thetransmitted and received signals, light absorption by the tissue may bedetermined. During each cardiac cycle, light absorption by the tissuevaries cyclically. During the diastolic phase, absorption is caused byvenous blood, non-pulsating arterial blood, cells and fluids in tissue,bone, and pigments. The level of light transmitted at the end of thediastolic phase is typically referred to as the “DC component” of thetotal light transmission. During the systolic phase, there is anincrease in light absorption (e.g., a decrease in transmitted light)compared with the diastolic phase due to the inflow of arterial bloodinto the tissue on which the pulse oximetry probe is attached. A crucialpulse oximetry principle is how the measurement can be focused on theblood volume representing the arterial blood only. In pulse oximetry,this is done by taking the pulsating arterial blood portion (the “ACsignal”) from the total transmission signal and normalizing this signalby the “DC” component. The resulting “AC/DC” signal is called the PPGwaveform. Pulse oximetry is thus based on the assumption that thepulsatile component of the absorbance is due to arterial blood only.

In pulse oximetry, arterial blood is typically modeled as containing twospecies of hemoglobin: oxyhemoglobin (HbO₂) and reduced hemoglobin (Hb).Oxyhemoglobin is hemoglobin that is fully saturated with oxygen, andreduced hemoglobin is without oxygen. Arterial oxygen saturationmeasured by pulse oximetry (SpO₂) is defined as the percentage of HbO₂divided by the total amount of hemoglobin (HbO₂+Hb). In order todistinguish between the two species of hemoglobin, light absorption ismeasured at two different wavelengths. The probe of a traditional pulseoximeter includes two different light sources, such as light-emittingdiodes (LEDs) or lasers, that emit light at two different wavelengths.The wavelength values widely used are 660 nm (red light) and 900 nm(infrared light), as the two species of hemoglobin have substantiallydifferent absorption at these wavelengths. Each light source isilluminated in turn at a frequency that is typically several hundred Hz.

Pulse oximeter sensors may be used in a variety of medical contexts,including continuous patient monitoring. During continuous patientmonitoring, output from a pulse oximeter sensor may be collected and/ordisplayed at a specified rate over a relatively long duration. Theduration of the continuous patient monitoring may vary, but in somecontexts the monitoring may occur for multiple hours or longer. Thus, itmay be desirable to configure the pulse oximeter as a remote sensor thatcommunicates wirelessly with a central unit, thereby allowing a patientundergoing monitoring to be untethered from wired connections andassociated bulky componentry. However, pulse oximeter sensors may demanda relatively high amount of power in order to drive the LEDs at theabove-described wavelengths and frequencies, thus limiting the abilityto configure the sensors as remote sensors. For example, batteries thatare small and/or inexpensive enough for use in a remote pulse oximetersensor may not store sufficient charge to power the pulse oximeter foran extended amount of time. Further, even in wired systems where amplecharge may be available, power consumption in the pulse oximeter may behigher than necessary, leading to wasted energy use, higher heatdissipation, or other issues.

Thus, according to embodiments disclosed herein, a transmitter andreceiver model may be applied to a pulse oximeter to account for noisesources that do not scale with the intensity of the light emitted by thelight emitters (e.g., LEDs) of the pulse oximeter. Examples of thenon-scaled noise sources include ambient light interferences and SpO₂sensor hardware-related noises, whereas motion interferences on the PPGwaveforms scale with the light intensity and, therefore, cannot bereduced by increasing light intensity. By reducing the non-scaled noisesources, the current applied to the light emitters may be reduced,thereby lowering power consumption of the pulse oximeter.

The transmitter and receiver model may measure/account for hardwarenoise present in the pulse oximeter, which may be determined before thepulse oximeter is used, such as during or immediately after manufactureof the pulse oximeter. The hardware noise may include random white noiseassociated with the LED driver, photodetector, analog to digitalconverter, and the analog front end. Then, when the pulse oximeter isaffixed to patient tissue, ambient noise and/or other environmentalnoise factors (such as electronic noise from nearby medical equipment)may be accounted for. The transmitter and receiver model may determinepower consumption of the LEDs as a function of signal to noise ratio(SNR), pulse frequency, pulse amplitude, duty cycle, sampling delays,pulse rise time, and/or other hardware-related parameters. During use ofthe pulse oximeter, a target SNR may be set and then model parametersmay be adjusted to minimize power consumption.

FIG. 1 is a block diagram of one embodiment of a multi-wavelength pulseoximetry system 10. Light transmitted from an emitter unit 100 passesinto patient tissue 102. The emitter unit includes multiple lightsources 101, such as light-emitting diodes (LEDs), with each lightsource having a dedicated wavelength. Each wavelength forms onemeasurement channel on which PPG waveform data are acquired. The numberof sources/wavelengths is at least two.

The light transmitted through the tissue 102 is received by a detectorunit 103, which comprises two photo detectors 104 and 105 in thisexample. For example, photo detector 104 may be a silicon photodiode,and photo detector 105 may be a second silicon photodiode with differentspectral characteristics or an indium gallium arsenide (InGaAs)photodiode. The emitter and detector units form a probe subunit 13 ofthe pulse oximetry system 10.

The probe subunit 13 may be coupled to a drive and processing subunit 11via a cable 107 and one or more connectors. For example, a connector maybe present on an end of cable 107 to connect cable 107 and probe subunit13 to drive and processing subunit 11. In this way, probe subunit 13 maybe removably coupled to drive and processing subunit 11. In otherexamples, probe subunit 13 and drive and processing subunit 11 mayintegrated into the same housing.

Drive and processing subunit 11 may include an input amplifier unit 106and an emitter drive unit 108. The photo detectors convert the opticalsignals received into electrical pulse trains and feed them to an inputamplifier unit 106. The amplified measurement channel signals arefurther supplied to a control unit 110, which executes instructionsstored in memory 112 to convert the signals into digitized format foreach wavelength channel.

The control unit 110 further controls emitter drive unit 108 toalternately activate the light sources. To activate the light sources,the emitter drive unit 108 may include a voltage source, such as abattery, which will be described in more detail below. As mentionedabove, each light source is typically illuminated several hundred timesper second. With each light source being illuminated at such a high ratecompared to the pulse rate of the patient, the control unit 110 obtainsa high number of samples at each wavelength for each cardiac cycle ofthe patient. The value of these samples varies according to the cardiaccycle of the patient, the variation being caused by the arterial blood.

The digitized PPG signal data at each wavelength may be stored in memory112 of the control unit 110 before being processed further according tonon-transitory instructions (e.g., algorithms) executable by the controlunit 110 to obtain physiological parameters. Memory 112 may comprise asuitable data storage medium, for example, a permanent storage medium,removable storage medium, and the like. Additionally, memory 112 may bea non-transitory storage medium. In some examples, the system 10 mayinclude a communication subsystem 117 operatively coupled to one or moreremote computing devices, such as hospital workstations, smartphones,and the like. The communication subsystem 117 may enable the output fromthe detector units (e.g., the digitized PPG signal data) to be sent tothe one or more remote computing devices for further processing, and/orthe communication subsystem may enable the output from the algorithmsdiscussed below (e.g., determined physiological parameters) to be sentto the remote computing devices. The communication subsystem 117 mayinclude wired and/or wireless communication devices compatible with oneor more different communication protocols. As non-limiting examples, thecommunication subsystem 117 may be configured for communication via awireless telephone network, a local- or wide-area network, and/or theInternet.

Algorithms may utilize the same digitized signal data and/or resultsderived from the algorithms and stored in the memory 112, for example.For example, for the determination of oxygen saturation and pulsetransit time (PTT), the control unit 110 is adapted to execute an SpO₂algorithm and a PTT algorithm, respectively, which may also be stored inthe memory 112 of the control unit 110. Additional algorithms, such as ablood pressure algorithm, a hypovolemia algorithm, and a respirationrate algorithm, may also be stored in memory 112 for determining bloodpressure, an indication of hypovolemia, and respiration rate,respectively. The obtained physiological parameters and waveforms may beshown on a screen of a display unit 114. Further, in some examples, thecontrol unit, memory, and/or other subsystems may be located remotelyfrom the rest of the sensor on a separate device, and the signal datafrom the detector units may be sent to the separate device forprocessing.

The input amplifier unit 106, the control unit 110 and memory 112, theemitter drive unit 108, probe subunit 13, and/or additional components(the display unit, for example) may collectively form a sensor. As usedherein, the term “probe” may refer to the probe and the attachment partsthat attach the optical components of the probe to the tissue site. Theterm pulse oximeter or pulse oximetry sensor may refer to a unitcomprising a probe, an analog front end, and a signal processing unitthat calculates SpO₂ and other blood characteristics. In amulti-parameter body area network system, the system typicallyrepresents a set of multiple sensors, e.g., the different physiologicalparameter measurements. Therefore, the whole measurement system maycomprise several sensors, and the sensors may communicate to a commonhub in which the parameters' information is integrated.

As used herein, the terms “sensor,” “system,” “unit,” or “module” mayinclude a hardware and/or software system that operates to perform oneor more functions. For example, a sensor, module, unit, or system mayinclude a computer processor, controller, or other logic-based devicethat performs operations based on instructions stored on a tangible andnon-transitory computer readable storage medium, such as a computermemory. Alternatively, a sensor, module, unit, or system may include ahard-wired device that performs operations based on hard-wired logic ofthe device. Various modules or units shown in the attached figures mayrepresent the hardware that operates based on software or hardwiredinstructions, the software that directs hardware to perform theoperations, or a combination thereof.

“Systems,” “units,” “sensors,” or “modules” may include or representhardware and associated instructions (e.g., software stored on atangible and non-transitory computer readable storage medium, such as acomputer hard drive, ROM, RAM, or the like) that perform one or moreoperations described herein. The hardware may include electroniccircuits that include and/or are connected to one or more logic-baseddevices, such as microprocessors, processors, controllers, or the like.These devices may be off-the-shelf devices that are appropriatelyprogrammed or instructed to perform operations described herein from theinstructions described above. Additionally or alternatively, one or moreof these devices may be hard-wired with logic circuits to perform theseoperations.

FIG. 2 schematically shows an example LED drive and detector circuit200, which may be included as part of pulse oximetry system 10 ofFIG. 1. LED drive and detector circuit 200 includes two light emitters,herein in the form of a first light emitting diode (LED) 202 and secondLED 204. When supplied with current at or above a threshold level, firstLED 202 emits light of a given wavelength range, such as a range between620 and 690 nm for red light. Second LED emits light in different range,such as in a range between 760 nm and 950 nm for infrared light. Battery206 may be selectively couplable to the drive circuit to provide a drivevoltage for illuminating the LEDs. As shown, battery 206 is coupled to avoltage regulator. The voltage regulator may be a two-channel, switchingregulator. For example, the voltage regulator includes a first channel(voltage regulator CH1 208) and a second channel (voltage regulator CH2210). Voltage regulator CH1 208 and voltage regulator CH2 210 may becontrolled according to signals received from timing unit 222 (includedas part of an analog front end (AFE) 230).

The circuit between voltage regulator CH1 and first LED 202 includes afirst bulk capacitor 212. Likewise, the first circuit between voltageregulator CH2 and second LED 204 includes a second bulk capacitor 214.Each of the bulk capacitors may be charged when the respective voltageregulator channel is turned on. When a respective LED is commanded on(e.g., commanded to illuminate), current may be supplied to the LED fromthe bulk capacitor and the respective voltage regulator channel may beturned off.

Pulse control of the LEDs may be provided by an LED control unit 216,which may be part of the AFE 230. LED control unit 216 may include avoltage controller 218 and a current regulator 220. For example, voltagecontroller 218 may include an input to receive pulse width modulation(PWM) data representative of what times during a corresponding PWM cycle(or other duration) LED 202 and LED 204 are to be activated. Voltagecontroller 218 may further include additional inputs to receive LEDcurrent (e.g., from current regulator 220) and a current regulatorvoltage headroom measurement. Voltage controller 218 may additionallyreceive manufacturing data indicative of certain parameters of the drivecircuit/pulse oximeter determined during manufacture (such as theforward voltage of each LED, any cable or connector resistances, etc.).Voltage controller 218 may adjust the output voltage based on themanufacturing data, voltage headroom, LED current, and/or other factors,as described in more detail below.

Timing unit 222 may output a signal to disable the voltage output fromthe voltage regulator CH1 208 and CH2 210 during LED pulses (e.g., whenLED 202 or LED 204 is illuminated). Timing unit 222 may also send asignal to current regulator 220 to activate/deactivate LED 202 or LED204. Current regulator 220 is configured to maintain the current I1flowing through LED 202 at or near a desired current (e.g., 100 mA) whenactive and maintain the current I2 flowing through LED 204 at or near adesired current (e.g., 100 mA) when active.

AFE 230 may include further components, including an analog to digitalconverter 224, high-frequency and/or low-frequency oscillators, andinput/output ports to communicate with the voltage regulator and with amemory (e.g., memory 112). AFE 230 further includes a detector circuitthat includes a receive channel 226 and a photo detector 228. Photodetector 228 is a non-limiting example of photo detector 104 and/or 105.Photo detector 228 may detect light that is emitted from LED 202 and/or204 (and that passes through intervening tissue) and send signalsindicative of the received light to receive channel 226.

In order to lower power consumption of the pulse oximeter to a levelwhere sufficiently long operation of the pulse oximeter may occur beforethe battery needs to be replaced or charged (e.g., a battery life of 48hours), a transmitter and receiver noise model may be provided that onlytakes into account noise sources that do not scale with the emittedlight intensity. Certain types of noise that may affect PPG signalquality, including patient motion and physiological events such as heartarrhythmias, scale with LED light intensity. However, accounting forthese sources of signal quality noise may not help lower powerconsumption due to the signal quality being the same before and afterthe change of the light intensity.

In contrast, the transmitter and receiver model described hereinmeasures and accounts for hardware noise and ambient light noise. Thehardware noise may be characterized before the pulse oximeter is used(e.g., while the pulse oximeter is still at the factory duringmanufacture). Ambient light effects and limits may also be characterizedbefore pulse oximeter use. For example, each of the hardware noise andthe ambient light effects may be characterized for different pulseoximeter types.

In particular, the approach described herein adjusts various modelparameters that impact power consumption (e.g., LED current amplitude,pulse length, and frequency) based on a current transfer ratio (CTR)rather than signal quality. The current transfer ratio is a measurementof the attenuation of the tissue being measured (e.g., a finger or earof a patient). By knowing the CTR, the hardware parameters that impactpower consumption may be adjusted in a manner that consumes a lowestamount of power while still providing a suitable signal quality. Duringcontinuous use of the pulse oximeter, the LED current pulse amplitudemay be adjusted to maintain a target SNR. The transmitter and receivermodel may be based on a shot noise model that characterizes SNR as afunction of the number of photons received at the detector. During useof the pulse oximeter, the CTR may be measured and a target SNR may bedetermined. The CTR and target SNR may be entered as inputs to the modelin order to determine an average LED current (for each LED), whichdefines the number of photons that are to be received at the detector inorder to generate a signal having the target SNR for the measured CTR.Based on the average LED current, the frequency and pulse length may bedetermined. Finally, the peak current (pulse amplitude) may becalculated based on the average LED current, frequency, and pulselength. During continuous use of the pulse oximeter on a patient (at thesame tissue site), if the target SNR is not reached, the pulse amplitudemay be adjusted. If a maximum LED current is reached or other hardwarelimits are hit, the sampling frequency may be adjusted and/or the pulselength may be adjusted to maintain the target SNR. If such adjustmentsare not sufficient to increase the SNR, the target SNR may be lowered.

Turning now to FIG. 3, a flow chart illustrating a method 300 forcharacterizing hardware noise in a pulse oximeter is shown. Method 300and the other methods described herein may be performed with a pulseoximetry system, such as pulse oximetry system 10 shown in FIG. 1. Morespecifically, method 300 may be executed by a control unit of the pulseoximetry system (such as control unit 110 shown in FIG. 1) according toinstructions stored on a non-transitory memory of the system (e.g.,memory 112 shown in FIG. 1) in combination with the various signalsreceived at the control unit from the system components and actuationsignals sent from the control unit to the emitter drive unit, inputamplifier unit, etc. Method 300 may be performed during or followingmanufacture of the pulse oximeter (e.g., at a factory where the pulseoximeter is manufactured and before sale or use of the pulse oximeter).However, in other examples, method 300 may be performed at other times,such as during use of the pulse oximeter.

At 302, method 300 includes characterizing and modeling the transmitterand receiver of the pulse oximeter based on LED power consumption. Powerconsumption (P) of each LED is a function of the signal to noise ratio(SNR) of the LED/detector, LED pulse frequency, pulse amplitude and dutycycle, sampling delays, detector pulse rise time, and other hardwarerelated parameters (including ambient light and control “hard limits”for gains and average LED current parameters) and system relatedparameters (such as sleep times and ADC sampling characteristics). Tocharacterize the noise generated by the transmitter and receiver, theSNR as a function of average LED current is measured for a plurality ofaverage LED currents, at a plurality of different current transferratios. The current transfer ratio is a ratio of collector current(e.g., photodetector current) to input current (e.g., LED current).Different CTRs may be generated by placing material/components withdifferent attenuation between the emitter and detector units of thepulse oximeter. The SNR may be determined by analyzing the signal outputfrom the detector, such as by determining the root mean square noise ofthe detector output signal and comparing it to the detector outputsignal amplitude. In some examples, the SNR may additionally oralternatively include measuring photodetector output during an LED pulse(the signal) and measuring photodetector output when the LED is off (thenoise). As an example, characterizing and modeling the transmitter andreceiver of the pulse oximeter based on LED power consumption may beperformed as a type test (e.g., for each sensor type or model) such thatthe characterizing and modeling is performed on a single sensor unit andapplied to all units of the same type (or model).

For example, after manufacture of the pulse oximeter, a first materialhaving a first attenuation may be placed between the LEDs and detectorof the pulse oximeter. The first material may have an attenuation knownto generate a current transfer ratio (CTR) of 10 nA/mA. The first LED(e.g., red LED) may be illuminated with a first amount of averagecurrent, such as 0.1 mA, and the SNR at the first amount of averagecurrent measured and recorded. The SNR may be measured and recorded fora plurality of different average currents, such as ten or fifteenaverage currents between 0.1 and 10 mA. A first curve of SNR versusaverage LED current for the first LED may be generated and stored inmemory. The process may be repeated for a plurality of differentmaterials having different attenuation, such as a second material togenerate a CTR of 20 nA/mA during a single LED pulse, a third materialto generate a CTR of 30 nA/mA during a single LED pulse, etc. Forexample, ten or twenty curves may be generated with CTRs ranging from10-100 nA/mA (e.g., if the pulse oximeter is configured to measurefinger profusion) or from 50-200 nA/mA (e.g., if the pulse oximeter isconfigured to measure ear profusion). The process may be repeated forthe second LED (e.g., the infrared LED). The average LED current may bea function of peak LED current (e.g., pulse amplitude), pulse width, andpulse frequency. Thus, to vary the average LED current, the pulseamplitude may be varied, the pulse width may be varied, and/or the pulsefrequency may be varied.

Method 300 may also include characterizing and modeling othertransmitter and receiver characteristics and ambient light at 306. Forexample, as indicated at 308, the detector pulse rise time may bemeasured and stored in memory. The pulse rise time may include the timefrom when the LED is activated until the signal output from the detectorreaches a maximum level. The signal output from the detector maygenerally be a square-wave signal, but may include a slow rise time atthe receiver and a delay from when light begins impinging on thedetector to when the maximum signal level is reached. Method 300 thenreturns.

FIG. 4 is a flow chart illustrating a method 400 for lowering powerconsumption of a pulse oximeter during use of the pulse oximeter. Method400 may be performed at least partially during an initialization phaseduring use of the pulse oximeter. For example, method 400 may beperformed in response to a pulse oximeter being powered on and the probesubunit (e.g., the LEDs and detector) being affixed to a sample, such asto a patient's finger, toe, or ear. Thus, method 400 may be performedeach time the pulse oximeter is powered on.

At 402, method 400 includes setting the LED current (I_(LED)) to aset-point current and measuring the output from the detector (detectorcurrent, I_(PD)) to determine the CTR of the probe for each LED. Theset-point current may be a suitable current, such as 20 mA, and mayrepresent the peak current (also referred to as the pulse amplitude).The detector current may be measured for each LED. The detector currentmay be calculated using a gain (for the AFE) that is set so that thedetector signal is at about half the maximum range of the analog todigital converter (ADC). The detector current is then calculated (foreach of the LEDs) using the set gain. The CTR is then calculated(CTR=I_(PD)/I_(LED)) for each LED.

At 404, method 400 includes setting an SNR target and determining anaverage LED current (I_(LED, AVG)) for each LED at the measured CTRbased on the stored transmitter characterization. The SNR target may bea predetermined SNR that minimizes error in the SpO₂ measurement, suchas an error of 1% or less. At 406, method 400 includes fixing LED driveparameters (e.g., drive current parameters) based on the transmitter andreceiver model and the SNR target. The LED drive parameters may include,for each LED, the LED pulse amplitude, sampling pulse width, andsampling frequency.

Accordingly, fixing the LED drive parameters includes, as indicated at408, obtaining a target average LED current for the target SNR for thesensor CTR from a stored characterization curve. The stored transmittercharacterization includes the plurality of curves generated atmanufacture of the pulse oximeter as described above with respect toFIG. 3. The curve for each LED that was generated at (or nearest to) themeasured CTR is selected and the I_(LED, AVG) is determined at thetarget SNR. For example, referring to FIG. 5, an example diagram 500showing curves of SNR versus average LED current (for a red LED, forexample) for a CTR of 20 nA/mA is shown. A first curve 502 shows astatistically best possible SNR versus average LED current based on ashot noise model. Curves 504 and 506 show curves generated by measuringSNR at different average currents for two different pulse oximeters. Asappreciated by curves 504 and 506, the SNR increases relatively linearlywith average LED current, at least at higher currents (e.g., above 0.5mA). An example target SNR may include 95 dB. Thus, for the first pulseoximeter, as shown by curve 504, the average LED current may be 3.2 mA(average LED current is depicted in logarithmic scale on the x axis).The average LED current for the second pulse oximeter (shown by curve506) may be higher, such as 5 mA.

The average LED current is a function of current amplitude, width, andfrequency. The approach described herein sets the pulse width andfrequency during initialization and then adjusts the pulse amplitude tomaintain the average current, and hence the target SNR. Accordingly,fixing the LED drive parameters includes determining a samplingfrequency that minimizes environmental noise, as indicated at 410.Further, fixing the LED drive parameters includes determining a pulsesample length, as indicated at 412.

The sampling frequency may be determined by measuring detector noise ata plurality of different sampling frequencies, such as for ambient lightreduction. For example, while the red LED and infrared LED are both off,the signal from the detector may be sampled at a plurality of differentfrequencies, ranging from 200 and about 3000 Hz, such as between 1500and 2500 Hz. Signal noise may be determined at each frequency using asuitable determination, such as by determining a root-mean-square (RMS)value of noise estimating a mean signal level, subtracting the meansignal from the instantaneous signal, squaring the difference, summingthe squared values over a measurement period, and taking the square rootof the value. The frequencies may be scanned until a downward trend innoise is detected. When the trend reverses, the frequency with thelowest noise is selected as the sampling frequency. However, othermethods for scanning the possible frequencies can be used. For example,every available frequency may be checked, and the one having the lowestnoise value may be selected as the sampling frequency. As used herein,sampling frequency may be equivalent to the LED pulse frequency, as thecontroller uses the same frequency to distinguish the photodetectorsignal corresponding to the red LED from the photodetector signalcorresponding to the infrared LED.

In some examples, downsampling of the raw samples may be performed untilthe final bandwidth of the plethysmographic pulse wave is reached, whichis typically around 100 Hz, such as 109 Hz. Further, additionally oralternatively, the plethysmographic pulse wave may be downsampled forradio communication (e.g., when the pulse oximeter is wireless andcommunicates the plethysmographic/SpO2 signals wirelessly via radiocommunication).

The pulse sample length may be determined based on the ADC oversamplingnoise reduction target and the target SNR. Further, the start of thesampling may be based on the photodetector rise time, which may bedetermined during manufacture of the pulse oximeter as described above.The sampling may commence when the pulse signal reaches 95% of themaximum pulse signal, thereby eliminating confounding noise during therise time when the pulse signal may be relatively weak.

Thus, the duty cycle sampling frequency and pulse length, as well as theLED average current for each LED, are determined. In order to maintainthe target SNR during continuous use of the pulse oximeter, the LEDcurrent (pulse amplitude) may be adjusted, for example, as ambient lightchanges. Accordingly, method 400 includes adjusting LED current tomaintain the target SNR at 414. As noise increases, for example, thesignal may be boosted by increasing LED current. In one example,initially the LED current for the first LED (e.g., the red LED) and theLED current for the second LED (e.g., the infrared LED) may be adjustedto produce approximately the same signal levels at the ADC (e.g., thesame signals output from the photodetector).

At 416, method 400 includes lowering the target SNR if one or morecontrol parameters hits a hardware limit. In one example, the target SNRmay only be lowered if repeating some of the above steps does not act toincrease the SNR. For example, if the LED current is adjusted as much aspossible (e.g., reaches a limit above which damage to the LED mayoccur), the CTR may be measured again (to ensure the attenuation of thetissue between the LEDs and detector has not changed). If the CTR haschanged, a new target average LED current may be selected and the LEDdrive parameters may be set as described above. If the CTR has notchanged and hence the target average LED current is the same, theoptimal sampling frequency may have changed (e.g., due to new noise inthe system, such as a change in ambient light, introduction ofadditional noise from nearby equipment, etc.). Thus, a new search for anoptimal sampling frequency may be conducted. If the optimal samplingfrequency does not change, the pulse length may be adjusted. Forexample, if a large amount of ambient light noise is present, thesampling pulse length may be decreased. If the pulse length isdecreased, the peak LED current may be increased to maintain a constantemitter pulse area. If the pulse length reaches its minimum (e.g., 15msec) and the target SNR is still not reached, the target SNR may belowered, for example to 90 dB, and the above steps repeated (e.g., a newaverage LED current may be selected, a new optimal sampling frequencydetermined, and a new pulse length determined). Method 400 then returns.

In this way, various pulse oximeter sensor model parameters that impactpower consumption (e.g., LED current amplitude, pulse length, andfrequency) may be adjusted based on a CTR rather than signal quality inorder to maintain a target SNR. For example, an average LED current (foreach LED) may be determined based on the CTR and the target SNR in orderto receive an appropriate number of photons at a photodetector of thesensor to achieve the target SNR. By adjusting the LED current based onthe SNR and the CTR, LED power consumption may be reduced.

While the hardware noise is described above as being characterized basedon SNR as a function of average LED current, in some examples, thehardware noise may be characterized based on average detector current.For example, in method 300 described above, SNR vs. average LED currentis measured and stored for a plurality of CTRs. Instead, or in addition,curves plotting SNR as a function of average detector current at aplurality of CTRs may be generated. To obtain different average detectorcurrents, the average LED current may be adjusted. Then, duringinitialization (e.g., according to the method of FIG. 4), a targetaverage detector current is determined based on the target SNR at themeasured CTR, rather than (or in addition to) the target LED current.The LED drive parameters may be adjusted to reach the target averagedetector current.

As an example, the methods described herein provide for characterizinghardware noise of an optical sensor as a function of average detectorcurrent and illuminating a light emitter of the optical sensor accordingto set sensor parameters, where the sensor parameters are set based onthe hardware noise characterization and the average detector current ofthe optical sensor and include current drive parameters of the lightemitter. Further, the current drive parameters of the light emitter areadjusted to maintain a target signal to noise ratio of the signal outputby the optical sensor.

While the methods were described above as being applicable to a pulseoximeter sensor, it is to be appreciated that the transmitter receivermodel may be applied to other types of optical sensors, such asmulti-wavelength optical sensors that may include more than two LEDsand/or that may output more than two wavelengths of light (e.g., opticalsensors configured to measure total hemoglobin and/or other bloodparameters), sensors other than pulse oximeter sensors that outputplethysmographic pulse waveforms, and so forth.

The technical effect of adjusting LED current based on a desiredsignal-to-noise ratio and a current transfer ratio is that LED powerconsumption may be reduced.

An example provides a method for an optical sensor, includingilluminating a light emitter of the optical sensor according to setsensor parameters, the sensor parameters set based on hardware noise orexternal interference characterization and light transmission orreflection of a tissue contributing to a signal output by the opticalsensor, the sensor parameters including current drive parameters of thelight emitter; and adjusting the current drive parameters of the lightemitter to maintain a target signal to noise ratio of the signal outputby the optical sensor. In a first example of the method, the opticalsensor is a pulse oximeter sensor, a multi-wavelength optical sensor, ora plethysmographic pulse waveform sensor. In a second example of themethod, which optionally includes the first example, the signal outputfurther includes external interference noise from ambient light. In athird example of the method, which optionally includes one or both ofthe first and second examples, the noise characterization is performedduring a development or a manufacturing phase of the optical sensor. Ina fourth example of the method, which optionally includes one or more oreach of the first through third examples, the external interferencenoise is characterized during an initialization phase of the opticalsensor. In a fifth example of the method, which optionally includes oneor more or each of the first through fourth examples, the light emitteris a light emitting diode (LED), and the method further includescharacterizing the hardware noise by generating a plurality of curvesthat each plot the signal to noise ratio (SNR) of the signal output byoptical sensor as a function of average LED current, each curvegenerated at a different current transfer ratio of the optical sensor.In a sixth example of the method, which optionally includes one or moreor each of the first through fifth examples, the light emitter is alight emitting diode (LED), and the method further includescharacterizing the hardware noise by measuring a signal to noise ratio(SNR) of the signal output by optical sensor as a function of averagedetector current. In a seventh example of the method, which optionallyincludes one or more or each of the first through sixth examples, themethod further includes, during the initialization phase of the opticalsensor, measuring the current transfer ratio of the optical sensor,selecting a target SNR, and setting the current drive parameters toreach a target average LED current, the target average LED currentcorresponding to noise characterizations at different SNR targets and aplurality of measurements at different current transfer ratios. In aneighth example of the method, which optionally includes one or more oreach of the first through seventh examples, setting the current driveparameters comprises selecting a current pulse amplitude, a currentpulse length, and a current pulse frequency. In a ninth example of themethod, which optionally includes one or more or each of the firstthrough eighth examples, selecting the current pulse amplitude, thecurrent pulse length, and the current pulse frequency to maintain thetarget average LED current comprises selecting a current pulsefrequency, from among a plurality of possible pulse frequencies, thatcontributes a smallest amount of ambient noise to the signal. In a tenthexample of the method, which optionally includes one or more or each ofthe first through ninth examples, selecting the current pulse amplitude,the current pulse length, and the current pulse frequency to maintainthe target average LED current comprises selecting a current pulseamplitude, from among a plurality of possible pulse amplitudes, thatexceeds the external interference from ambient light. In an eleventhexample of the method, which optionally includes one or more or each ofthe first through tenth examples, selecting the current pulse amplitude,the current pulse length, and the current pulse frequency to maintainthe target average LED current comprises selecting the current pulselength based on the target SNR. In a twelfth example of the method,which optionally includes one or more or each of the first througheleventh examples, selecting the current pulse amplitude, the currentpulse length, and the current pulse frequency to maintain the targetaverage LED current comprises selecting the current pulse amplitudebased on the selected current pulse frequency, the selected currentpulse length, and the target average LED current.

Another example provides a method for a pulse oximeter sensor includinga light emitting diode (LED), the method including, during continuoussensor operation, adjusting LED current drive parameters to maintain atarget signal to noise ratio (SNR) of a signal output by the pulseoximeter sensor, the LED current drive parameters of the LED adjustedfrom base LED current drive parameters selected based on the target SNRand a measured current transfer ratio of the sensor. In a first exampleof the method, the method further includes selecting the base LEDcurrent drive parameters based on a target average LED current, thetarget average LED current obtained from a curve stored in memory of thesensor, the curve plotting SNR of the signal output by the pulseoximeter sensor as a function of average LED current and generated atthe measured current transfer ratio. In a second example of the method,which optionally includes the first example, the method further includesselecting the base LED current drive parameters based on a targetaverage detector current. In a third example of the method, whichoptionally includes one or both of the first and second examples, themethod further includes, during the continuous sensor operation,illuminating the LED at a pulse frequency and a pulse length, the pulsefrequency and the pulse length each selected to minimize a contributionof ambient noise to the signal output by the pulse oximeter sensor. In afourth example of the method, which optionally includes one or more oreach of the first through third examples, the method further includes,during the continuous sensor operation, processing the signal output bythe pulse oximeter sensor to provide a measurement of one or morephysiological parameters of a patient. In a fifth example of the method,which optionally includes one or more or each of the first throughfourth examples, the measured current transfer ratio defines a level ofsignal attenuation provided by the patient, and wherein, during thecontinuous sensor operation, the sensor is affixed to the patient.

Another example provides a system for an optical sensor, including alight emitter; a detector configured to output a signal representativeof light from the light emitter received at the detector; and a controlunit configured to: illuminate the light emitter according to setcurrent drive parameters, the current drive parameters set based onhardware noise contributing to the signal output by the detector, andfurther based on ambient noise contributing to the signal, the currentdrive parameters including a current pulse amplitude and duty cycle ofthe light emitter; and adjust the current drive parameters of the lightemitter to maintain a target signal to noise ratio of the signal outputby the detector.

Another example provides a method for an optical sensor, includingcharacterizing hardware noise of the optical sensor as a function ofaverage detector current, illuminating a light emitter of the opticalsensor according to set sensor parameters, the sensor parameters setbased on the hardware noise characterization and the average detectorcurrent of the optical sensor, the sensor parameters including currentdrive parameters of the light emitter, and adjusting the current driveparameters of the light emitter to maintain a target signal to noiseratio of the signal output by the optical sensor.

As used herein, an element or step recited in the singular and proceededwith the word “a” or “an” should be understood as not excluding pluralof said elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. Moreover, unlessexplicitly stated to the contrary, embodiments “comprising,”“including,” or “having” an element or a plurality of elements having aparticular property may include additional such elements not having thatproperty. The terms “including” and “in which” are used as theplain-language equivalents of the respective terms “comprising” and“wherein.” Moreover, the terms “first,” “second,” and “third,” etc., areused merely as labels and are not intended to impose numericalrequirements or a particular positional order on their objects.

This written description uses examples to disclose the invention,including the best mode, and also to enable a person of ordinary skillin the relevant art to practice the invention, including making andusing any devices or systems and performing any incorporated methods.The patentable scope of the invention is defined by the claims and mayinclude other examples that occur to those of ordinary skill in the art.Such other examples are intended to be within the scope of the claims ifthey have structural elements that do not differ from the literallanguage of the claims or if they include equivalent structural elementswith insubstantial differences from the literal languages of the claims.

1. A method for an optical sensor, comprising: illuminating a lightemitter of the optical sensor according to set sensor parameters, thesensor parameters set based on hardware noise or external interferencecharacterization and light transmission or reflection of a tissuecontributing to a signal output by the optical sensor, the sensorparameters including current drive parameters of the light emitter; andadjusting the current drive parameters of the light emitter to maintaina target signal to noise ratio of the signal output by the opticalsensor.
 2. The method of claim 1, wherein the optical sensor is a pulseoximeter sensor, a multi-wavelength optical sensor, or aplethysmographic pulse waveform sensor.
 3. The method of claim 1,wherein the signal output further includes external interference noisefrom ambient light.
 4. The method of claim 3, wherein the noisecharacterization is performed during a development or a manufacturingphase of the optical sensor.
 5. The method of claim 3, wherein theexternal interference noise is characterized during an initializationphase of the optical sensor.
 6. The method of claim 5, wherein the lightemitter is a light emitting diode (LED), and further comprisingcharacterizing the hardware noise by measuring a signal to noise ratio(SNR) of the signal output by optical sensor as a function of averagedetector current.
 7. The method of claim 5, wherein the light emitter isa light emitting diode (LED), and further comprising characterizing thehardware noise by generating a plurality of curves that each plot asignal to noise ratio (SNR) of the signal output by the optical sensoras a function of average LED current, each curve generated at adifferent current transfer ratio of the optical sensor.
 8. The method ofclaim 7, further comprising, during the initialization phase of theoptical sensor, measuring the current transfer ratio of the opticalsensor, selecting a target SNR, and setting the current drive parametersto reach a target average LED current, the target average LED currentcorresponding to noise characterizations at different SNR targets and aplurality of measurements at different current transfer ratios.
 9. Themethod of claim 8, wherein setting the current drive parameterscomprises selecting a current pulse amplitude, a current pulse length,and a current pulse frequency.
 10. The method of claim 9, whereinselecting the current pulse amplitude, the current pulse length, and thecurrent pulse frequency to maintain the target average LED currentcomprises selecting a current pulse frequency, from among a plurality ofpossible pulse frequencies, that contributes a smallest amount ofambient noise to the signal.
 11. The method of claim 9, whereinselecting the current pulse amplitude, the current pulse length, and thecurrent pulse frequency to maintain the target average LED currentcomprises selecting a current pulse amplitude, from among a plurality ofpossible pulse amplitudes, that exceeds the external interference fromambient light.
 12. The method of claim 9, wherein selecting the currentpulse amplitude, the current pulse length, and the current pulsefrequency to maintain the target average LED current comprises selectingthe current pulse length based on the target SNR.
 13. The method ofclaim 12, wherein selecting the current pulse amplitude, the currentpulse length, and the current pulse frequency to maintain the targetaverage LED current comprises selecting the current pulse amplitudebased on the selected current pulse frequency, the selected currentpulse length, and the target average LED current.
 14. A method for apulse oximeter sensor including a light emitting diode (LED),comprising: during continuous sensor operation, adjusting LED currentdrive parameters to maintain a target signal to noise ratio (SNR) of asignal output by the pulse oximeter sensor, the LED current driveparameters of the LED adjusted from base LED current drive parametersselected based on the target SNR and a measured current transfer ratioof the sensor.
 15. The method of claim 14, further comprising selectingthe base LED current drive parameters based on a target average LEDcurrent, the target average LED current obtained from a curve stored inmemory of the sensor, the curve plotting SNR of the signal output by thepulse oximeter sensor as a function of average LED current and generatedat the measured current transfer ratio.
 16. The method of claim 14,further comprising selecting the base LED current drive parameters basedon a target average detector current.
 17. The method of claim 14,further comprising, during the continuous sensor operation, illuminatingthe LED at a pulse frequency and a pulse length, the pulse frequency andthe pulse length each selected to minimize a contribution of ambientnoise to the signal output by the pulse oximeter sensor.
 18. The methodof claim 14, further comprising, during the continuous sensor operation,processing the signal output by the pulse oximeter sensor to provide ameasurement of one or more physiological parameters of a patient. 19.The method of claim 18, wherein the measured current transfer ratiodefines a level of signal attenuation provided by the patient, andwherein, during the continuous sensor operation, the sensor is affixedto the patient.
 20. An optical sensor, comprising: a light emitter; adetector configured to output a signal representative of light from thelight emitter received at the detector; and a control unit configuredto: illuminate the light emitter according to set current driveparameters, the current drive parameters set based on hardware noisecontributing to the signal output by the detector, and further based onambient noise contributing to the signal, the current drive parametersincluding a current pulse amplitude and duty cycle of the light emitter;and adjust the current drive parameters of the light emitter to maintaina target signal to noise ratio of the signal output by the detector. 21.A method for an optical sensor, comprising: characterizing hardwarenoise of the optical sensor as a function of average detector current;illuminating a light emitter of the optical sensor according to setsensor parameters, the sensor parameters set based on the hardware noisecharacterization and the average detector current of the optical sensor,the sensor parameters including current drive parameters of the lightemitter; and adjusting the current drive parameters of the light emitterto maintain a target signal to noise ratio of the signal output by theoptical sensor.